default response_callback and finished_callback to an empty deque instead of None

fixes #1600

3 files changed, 16 insertions, 15 deletions

diff --git a/CHANGES.txt b/CHANGES.txt
index a9e159275..87e9f1f3a 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -191,6 +191,10 @@ Bug Fixes
 - Avoiding timing attacks against CSRF tokens.
   See https://github.com/Pylons/pyramid/pull/1574
 
+- ``request.finished_callbacks`` and ``request.response_callbacks`` now
+  default to an iterable instead of ``None``. It may be checked for a length
+  of 0. This was the behavior in 1.5.
+
 Deprecations
 ------------
 
diff --git a/pyramid/request.py b/pyramid/request.py
index 13b8cd339..5739e6311 100644
--- a/pyramid/request.py
+++ b/pyramid/request.py
@@ -37,8 +37,14 @@ class TemplateContext(object):
     pass
 
 class CallbackMethodsMixin(object):
-    response_callbacks = None
-    finished_callbacks = None
+    @reify
+    def finished_callbacks(self):
+        return deque()
+
+    @reify
+    def response_callbacks(self):
+        return deque()
+
     def add_response_callback(self, callback):
         """
         Add a callback to the set of callbacks to be called by the
@@ -76,11 +82,7 @@ class CallbackMethodsMixin(object):
             See also :ref:`using_response_callbacks`.
         """
 
-        callbacks = self.response_callbacks
-        if callbacks is None:
-            callbacks = deque()
-        callbacks.append(callback)
-        self.response_callbacks = callbacks
+        self.response_callbacks.append(callback)
 
     def _process_response_callbacks(self, response):
         callbacks = self.response_callbacks
@@ -135,12 +137,7 @@ class CallbackMethodsMixin(object):
 
             See also :ref:`using_finished_callbacks`.
         """
-
-        callbacks = self.finished_callbacks
-        if callbacks is None:
-            callbacks = deque()
-        callbacks.append(callback)
-        self.finished_callbacks = callbacks
+        self.finished_callbacks.append(callback)
 
     def _process_finished_callbacks(self):
         callbacks = self.finished_callbacks
diff --git a/pyramid/tests/test_request.py b/pyramid/tests/test_request.py
index 2c2298f26..c528b9174 100644
--- a/pyramid/tests/test_request.py
+++ b/pyramid/tests/test_request.py
@@ -120,7 +120,7 @@ class TestRequest(unittest.TestCase):
 
     def test_add_response_callback(self):
         inst = self._makeOne()
-        self.assertEqual(inst.response_callbacks, None)
+        self.assertEqual(len(inst.response_callbacks), 0)
         def callback(request, response):
             """ """
         inst.add_response_callback(callback)
@@ -171,7 +171,7 @@ class TestRequest(unittest.TestCase):
 
     def test_add_finished_callback(self):
         inst = self._makeOne()
-        self.assertEqual(inst.finished_callbacks, None)
+        self.assertEqual(len(inst.finished_callbacks), 0)
         def callback(request):
             """ """
         inst.add_finished_callback(callback)